"Space Mountain (and many coasters) use what is called a Zone-Block system. It basically sections off portions of the track to allow one to operate more cars on the track at a time. Increasing capacity, and preventing possible collisions should a car stop somehow. Each zone features a brake at the beginning. As long as there is a car in the zone, the brake is closed, preventing another car from entering the zone.
The downside to running many cars on the track is that there are only so many spots in the loading area. In the case of Space Mountain there are 6 slots in the loading area, but they can run up to 10 rockets.
If something prevents the employees from moving rockets in the loading area (a very slow loading person)... the free slots fill up in the station, and the cars on the ride have no where to go.
Hence they hit the brake in zone 15 and stop. Then Zone 14 cant go into zone 15, so zone 14 stops.... and so on.
This is what you see in the pic here."